The present invention relates to the use of a mixture of polio esters as insecticides to eliminate or reduce plant pests. More particularly, this invention concerns a mixture of sugar esters which have insecticidal activity and which are environmentally friendly.
The following description of the background of the invention is provided to aid in understanding the invention, but is not admitted to be, or to describe, prior art to the invention. All publications are incorporated by reference in their entirety.
Sucrose octanoate has proven to be a useful insecticide compound. Varieties of sucrose esters are contained in the natural wax of leaves. Discussions of these esters may be found, for example, in Neal, J. W. Jr. et al, J. Econ. Entomol. 87, 1600-1607(1994); Puterka, G. J., et al, J. Econ. Entomol. 88, 615-619(1995), and Lui, T. X. et al, J. Econ. Entomol. 89, 1233-1239 (1996). Sucrose octanoate is contained in the mixture of sucrose esters made when coconut fatty acids are used to make sucrose esters. The sucrose esters are readily biodegradable and hydrolyze to readily metabolizable sucrose and fatty acid. Sucrose esters can be made by the methods disclosed in U.S. Pat. 5,756,716, William A. Farone and Robert Serfass, xe2x80x9cMethod for Production of Sugar Estersxe2x80x9d, May 26, 1998. Other methods for making these compounds are also known and referenced in this patent.
The efficient production of sucrose octanoate involves several steps, including an esterification, a transesterification and then a purification step. It would be extremely useful to have compounds with similar insecticidal activity, similar environmental acceptability, made from similar natural products, that could be synthesized in fewer steps. Unfortunately there is no means of predicting the chemical structures that will have insecticidal activity. There is no general agreement as to exactly how the sugar ester compounds obtain their insecticidal activity.
One hypothesis is that the compounds like sucrose laurate or sucrose octanoate act as surfactants to dewax the insect""s protective coating. The insect then either dehydrates or is readily attacked by microbes. This hypothesis is supported by the observation that the compounds are xe2x80x9ccontactxe2x80x9d insecticides. Since the sucrose esters are constituents of plant leaves, there is another hypothesis that the compounds somehow interfere with the metabolism of the insect to prevent them from eating the tissue that the esters protect. This hypothesis requires ingestion of the material by the insect and cannot be ruled out since xe2x80x9ccontactxe2x80x9d can also result in ingestion.
It is also known that the short chain sucrose esters that are effective as insecticides have certain properties that seem to enhance that activity. Chortyk and co-workers at the United States Department of Agriculture [see Chortyk, O. T., Pomonis, J. G., and Johnson, A. W., J. Agric. Food Chem., 44, 1551-1557 (1996)] concluded that the sucrose esters with fatty acid chain lengths below 12 were more effective especially when there were 2 or 3 side chains on the sucrose. The fact that there are eight hydroxyl groups that can be esterified in sucrose means that, in principal, one can make 8 sucrose monoester, 28 diester and 56 triester isomers. It is unpredictable if all esters of one type (e.g. monoesters, diesters, etc.) are equally effective. Molecular orbital calculations performed in the inventors"" laboratory suggest that not all esters are equally likely to be produced during synthesis.
In accordance with the present invention and as used herein, the following terms are defined with the following meanings, unless explicitly stated otherwise.
The term xe2x80x9cenhancedxe2x80x9d refers to increasing or improving a specific property.
In one aspect the present invention relates to a new environmentally friendly polyol ester insecticide. The inventors unexpectedly found that a mixture of polyol esters has a greater insecticidal activity than the individual components or what one could reasonably anticipate from an additive effect.
More particularly another aspect of this instant invention is the use of these esters as safe effective insecticides. The inventors found the surprising and unexpected result that a mixture of octanoic acid (C8) esters is more effective as insecticides, and that a mixture of sucrose and sorbitol acid esters were the most effective.
Also there was the surprising finding that for sucrose octanoate mixture the use of an alcohol solvent including but not limited to such alcohols as butanol, propanol, ethanol, methanol, and the likes, added at low concentrations provided a composition that was shown to be even more effective as an insecticide than sucrose octanoate alone.
The method of preparation of the polyol esters, in particular sorbitol, of this invention is best explained in terms of 7 steps. One of the objects of the preparation method is have an environmentally acceptable synthesis that produces no toxic by-products. Without limiting the scope of this invention as expressed by the claims which follow, the synthesis steps will be discussed briefly.
The process is basically as follows:
1. The desired organic acid (e.g. octanoic, deconoic, but not limited to these) is charged to the reactor at a temperature sufficiently high to keep it in liquid form.
2. The polyol (e.g. either xylitol or sorbitol) is added in an amount that would allow the production of the monoester stoichiometrically plus an additional 10% to drive the reaction essentially to completion.
3. An esterification catalyst is added. Any usual catalyst can be used such as sulfuric acid or phosphoric acid. Phosphoric acid is the preferred embodiment in this case since neutralization at the completion of the reaction provides a phosphate salt that can either be left in the product (since phosphorous is an essential plant nutrient and phosphates are a known method of providing phosphorus) or removed by filtration if desired (whereupon the salt can be sold separately for fertilizer use).
4. The reactor is held at a temperature sufficiently high along with a pressure sufficiently low to allow water to be removed as the esterification reaction proceeds. For most of the esters a temperature around 150xc2x0 C. and atmospheric pressure was used.
5. The reaction is allowed to proceed until the remaining organic acid reaches a low equilibrium value. This point can be determined very simply by monitoring the free acid content of the reaction mixture and comparing differing reaction times (see Example 1 and 2). When the free organic acid is reduced no further the reaction is essentially completed. The equilibrium value in weight percent depends on the molecular weight of the organic acid and the structure of the isomers formed. Once determined for a particular organic acid and polyol combination it can be used as a measure of reaction completion.
6. At the completion of the reaction (approximately 18-30 hours for the esters synthesized for the insecticidal studies) the solution is neutralized with an amount of base that is sufficient to neutralize all of the mineral acid used as a catalyst plus bring the solution to a desired pH for subsequent use. If calcium hydroxide is used as the base, calcium phosphate can be filtered out of the product. Other bases could be used depending on the desired nature of the final product. This procedure was followed to allow for a product of good water solubility with little or no residual fine solid particles.
7. The product (filtrate from Step 6) is analyzed and is ready for use.
This procedure of this present invention is deliberately made deceptively simple. Due to the fact that the insecticide nature as well as other properties of these materials change depending on the isomers it is desired to have a simple process that can be repeated with little difficulty. The only xe2x80x9cwaste productxe2x80x9d of the reaction is the water removed during the esterification. The equipment and reaction conditions are selected in such a manner that the tendency of any of the organic acid to distill over with the water is thwarted by the use of appropriate reflux allowing the water to be removed and the acid to fall back into the reactor. Thus, in the preferred method a distillation column (tray or packed column) is used over the reactor to insure retention of the acids.
Sucrose octanoate is synthesized by the method described in U.S. Pat. 5,756,716, incorporated herein by reference. The resultant product, sucrose octanoate, is found to have monoesters that are more effective as insecticides than the diesters and triesters of sucrose octanoate. This finding is in contradiction to the finding of Chortyk. The inventors of the above described process find the method of synthesis is important in defining the distribution of isomers in complex molecules with the subsequent result that one must either specify the exact nature of the isomers involved and/or the method of synthesis as a mean of selecting the best insecticides.
In the sucrose studies the octanoate was found to be the approximately optimal chain length. Octanoic acid is a reasonably abundant fatty acid fraction of natural oils (e.g. coconut oil) after the oil is xe2x80x9csplitxe2x80x9d, i.e. hydrolyzed to glycerol and fatty acids. Nature prefers even chain fatty acids. Although the odd chain fatty acids are also likely to be reasonably effective it is the inventors"" purpose to make the biodegradation products as natural as possible. It is well known that the long chain fatty acid esters of sucrose (e.g. sucrose stearate) are extremely mild materials with excellent surfactant properties. These materials have been used as food emulsifiers for many years.
A wide variety of compounds were synthesized. The compounds that were proven to have the best activity when compared to sucrose octanoate are sorbitol and xylitol esters of short chain fatty acids, particularly the octanoic and decanoic acid monoesters. These compounds are more easily prepared than the sucrose octanoate. They can be synthesized directly from the raw materials in a single step using only a neutralizable mineral acid as a catalyst in the process described earlier. Due to the greater ease of synthesis these materials could be less expensive even if they are slightly less effective than sucrose octanoate.
The following are examples of making polyol esters according to the present invention. Other polyol esters may also be made using the process of this invention.
Examples 1 and 2 were run to compare different times for the degree of conversion. This type of benchmark reaction can be performed to determine optimal conditions for other polyol esters.